Chapter 21 — That God is his own essence

From what precedes we can conclude that God is his own essence, quiddity, or nature.

In everything which is not its own essence or quiddity there must be some kind of composition: for, since each thing contains its own essence, if a thing contained nothing besides its own essence, all that a thing is would be its essence. Therefore, if a thing is not its own essence, there must be something in it besides its essence. Consequently, there must be composition in it. For which reason the essence in composite things has the signification of a part, as humanity in a man. Now, it has been shown (Ch. 18.) that there is no composition in God. Therefore, God is his own essence.

Again. It seems that only that which does not enter into the definition of a thing is beside the essence of that thing, for a definition signifies what a thing is. (Aristotle, Metaphysics 4.8, 1012b5–11.) Now only the accidents of a thing do not enter into its definition, and consequently only accidents are in a thing besides its essence. But in God there are no accidents, as we shall show further on. (Ch. 23.) Accordingly, there is nothing in him besides his essence. Therefore, he is his own essence.

Moreover. Forms that are not predicated of subsistent things, whether the latter be taken universally or singly, are forms which are not, of themselves, single subsistent forms individualized in themselves. For we do not say that Socrates, or man, or an animal is whiteness, because whiteness is not singly subsistent in itself, but is individualized by its subsistent subject. Likewise, natural forms do not subsist singly in themselves, but are individualized in their respective matters: hence we do not say that this individual fire, or fire in general, is its own form. Moreover, the essences or quiddities of genera or species are individualized by the signate matter of this or that individual, even though the quiddity of a genus or species includes form and matter in general. Hence we do not say that Socrates, or man, is humanity. Now the divine essence exists singly in itself, and is individualized in itself, since it is not in any matter, as shown above. (Ch. 17.) Hence the divine essence is predicated of God, so that we say: God is his own essence.

Further. The essence of a thing is either the thing itself, or is related to it in some way as cause, since a thing derives its species from its essence. But nothing can in any way be a cause of God: for he is the first being, as shown above. (Ch. 13.) Therefore, God is his own essence.

Again. That which is not its own essence is related in respect of some part of itself to that essence, as potency to act. Therefore, the essence is signified by way of form (for instance, “humanity”). But there is no potency in God, as shown above. (Ch. 16.) Therefore, it follows that he is his own essence.